While I understand that it must annoy you, the story DID make it clear from the beginning that romance is not the focus of this manga but rather getting over emotional trauma and succeeding in being a celebrity to get revenge. The romance is just a side focus/additional cherry on top, really.
My problem is not only the development of their love story. Is the whole story in general. Kyoko is still 16. 16!!!??? She’s been 16 for 15 years. How can she truly develop as an artist with no growth as a person? No years of experience? This 18 year old story has been taking place only for a year!!! Kyoko needs to grow. Mature. Become an adult!! Stop being childish. That’s how you develop a character. That’s how you develop a person. You grow!!
You seem to forget tho that one chapter does not equal the happenings of even one day often enough, let alone months or years.
Not all stories are as rushed as what you describe. It doesn't need to take (canon) years for a character to develope at times and some never change at all either. That's the charm of some characters as well.
Kyoko's growth does actually not focus on physical aspects, it's her mental trauma that needs to be worked on. Nothing else. After that is dealt with, Ren's trauma needs to be solved and THEN there will be progress for them in a relationship.
Again, the focus of this manga was never to make her grow as a person or develope, she wants revenge by becoming a top actress (which will take a lot of time or a special happening at some point) and after that comes her romance issues.
As I said, one chapter often doesn't even full up one day in the manga and that's not actually a bad thing. Stories like these are much better if they are slow and natural paced without any large time skips or other crap that excuses lazy writing or just lack of ideas (tho I know that's a common thing for many authors out there). As for the pacing itself, if you re-read the whole thing you can notice how well paced it actually is. We as readers cannot expect the author to churn out one chapter a day or keep up real-life progress for a story like this. At least not without the quality of the art, story and overall enjoyment crumbling to pieces.
Either way, Kyoko DID actually grow. Both as a person and physically. She overcame quite a few issues, became less distrustful, made a name for herself as and actor and celebrated a birthday already. So I'm not quite sure why you think she has not grown at all.
Growing takes time and since the story is slow paced and intricate, as well as not focused on the love/romance aspect at all since the characters in question are literally not able to have a romance due to their mental issues, there is little to no room to complain. It was clear from the start that the issues she (and Ren) wants to overcome and the goals she has are very big ones, so they cannot be achieved in 12 chapters or so unless the author rushes through the basic plotline and generally doesn't care about detailed writing/developement.
Well I’m done. After 12 years of waiting, we got a confession and reciprocated feelings, only to end with: I have something do to first. A whole chapter of just sitting on a couch. Can’t take it anymore. Hope nothing happens to Sensei and that maybe in about 10 more years, you all get what you are all waiting for. Actual development of this love story.